Hitherto, a number of studies have been conducted to develop and easily produce a ceramic filter having excellent thermal and chemical properties and composed of cordierite or mullite as a main crystalline phase.
Arthur, et al. described in U.S. Pat. No. 3,954,672 a synthesis of cordierite by use of talc, clay, kyanite, alumina and water and the use thereof as materials for a catalyst carrier, turbine engine, heat exchanger or furnace that is not thermally transformable. In said patent, it is highlighted that the contents of Na2O or K2O should be less than 0.14%.
U.S. Pat. No. 3,940,255 (Roy, et al.) describes the use of small amount of Mo, Ta, Zr, Nb, Ti, Li, As, etc. as a nucleating agent to better embody the crystalline structure of cordierite. In said patent, metal oxides such as Mo, Ta, Zr, Nb, Ti, Li, As, etc. are not structural components, but additional components, which are added with a very small amount, i.e., 0.5-3%, where a thermal expansion coefficient of the cordierite synthesized therein is restricted to less than 1.54×10−6/K.
U.S. Pat. No. 4,042,403 (Richard, et al.) describes a synthesis of cordierite structure by using 3-5% of Li2O, 0.25-2.5% of MgO, 15-20% of Al2O3, 68-75% of SiO2 and 2-5.5% of TiO2. The structure of cordierite produced in said patent is proved to be thermally stable and shows 100 ppm or less of change in length when operated for 2000 hours at 950° C.
U.S. Pat. No. 4,528,275 (James, et al.) describes a multi-crystalline body consisting of 50-95% of mullite and 5-50% of cordierite. Said patent discloses, as a method for lowering a thermal expansion coefficient with maintaining the properties of mullite, a synthesis of materials whose thermal expansion coefficients are generally lower than a pure mullite by adding cordierite in the presence of TiO2 as a nucleating agent.
U.S. Pat. No. 4,921,616 (Louis, et al.) describes a preparation of mullite/zirconia composite having an excellent thermal resistance from SiO2, Al2O3 and ZrO2, and shows that a filter produced from such mullite/zirconia composite is durable even at a temperature of 1650° C. and thus can be used as a filter of melted metal, whereas a conventional single cordierite or a cordierite/mullite composite cannot be used at a high temperature higher than 1500° C.
U.S. Pat. No. 4,950,628 (Thomas, et al.) describes a synthesis of cordierite having a very low thermal expansion coefficient (7×10−6/K) by adding talc, kaolin, aluminum oxide, amorphous silica or the like to a SiO2—Al2O3—MgO precursor mixture for the preparation of cordierite. The cordierite synthesized as above is reported to be an excellent material in terms of thermal shock as well as thermal expansion coefficient.
However, cordierite having the composition of 2MgO-2Al2O3-5SiO2 is vulnerable to acid and base owing to the MgO component. Since MgO forms a crystalline structure together with Al2O3 and SiO2, it has some resistance to acid and base. However, when exposed to sulfur dioxide gas contained in automobile exhaust gas for an extended period of time, MgO transforms into MgSO4, which may weaken the overall structure of cordierite owing to MgSO4-'s weak strength. Therefore, in a catalyst system for the purification of automobile exhaust gas wherein heating and cooling repetitively occur and which is exposed to sulfur dioxide gas for an extended period of time, it is difficult to prevent a catalyst carrier comprising cordierite from the deterioration in strength and structure. Such problems cannot be overcome unless one or more components constituting cordierite ceramics is or are changed or substituted.
Therefore, it has been required in the art to develop a catalyst carrier or a filter material which is strongly resistant to acid and base with maintaining its excellent thermal properties. Further, said catalyst carrier or a filter material should be structurally stable and have durability in the gas containing acids or bases and its structure should be not weakened or destructed even after a long operation.
In order to produce the ceramic structure which is strong against acid and base, each component constituting the ceramic structure should have low reactivity with or high resistance to acids and bases. This is because, even if each oxide of components is crystallized or synthesized at a temperature higher than its melting point so as to form a material such as a composite oxide, the inherent chemical characteristics of each oxide do not disappear, and as a result, the strength of the overall structure of composite structure may be weakened.